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Why won't Mountain Lion run on some MacBook Pros?

I have an older MacBook Pro and was looking forward to upgrading the OS to Mountain Lion. I was at the local Apple store last week and while talking to one of the staff, I was told that there was a chance Mountain Lion wouldn't work with my MacBook, but she couldn't give me a clear answer as to why. I though that the main limitation was that since Mountain Lion is 64 bit, you just needed a Core 2 or later 64 bit processor. Well, my Mac is 64 bit and would seem to meet that requirement. What would keep Mountain Lion from running on some 64 bit Macs and not others?

I have a 2007 MacBook. The old plastic white one and I am currently running Mountain Lion. As far as I can tell its all about the hard drive in the older models. I recently replaced the one in my 2007 with a 1TB and not only does Mountain Lion work, it now reads as a late model 2009 in the "about this Mac" section. I upgraded the ram too but I doubt that has anything to do with it. Even if you replace your current hard drive with the same size, just newer, that should do you. Hope this helps.

I agree. Some of the early 64-bit Macs sometimes have 32-bit graphics drivers and that is causing the compatibility issue. If you can figure out whether you have a 32-bit or 64-bit EFI, you should be able to tell whether you are good to go or not. This is the command to run in Terminal that will tell you - if it returns "EFI32" you will have a problem running Mountain Lion: ioreg -l -p IODeviceTree | grep firmware-abi

"Apple declined to tell us the reasoning behind leaving some of these models out of potential Mountain Lion upgrades, but we suspected it is related to an updated graphics architecture that is designed to improve OS X's graphics subsystem going forward. Our own Andrew Cunningham suspects the issue is more specifically related to graphics drivers, since the GPUs not supported under Mountain Lion have drivers that were written before 64-bit support was common.

Information included with the first Mountain Lion GM now corroborates the connection to 32-bit graphics drivers as the culprit. While Mountain Lion is compatible with any Mac capable of running a 64-bit kernel, the kernel does not support loading 32-bit kernel extensions (KEXTs). Furthermore, Macs with older EFI versions that are not 64-bit clean won't load Mountain Lion's 64-bit only kernel.

As you might have already guessed, graphics drivers are KEXTs under OS X. And the GPUs in some of those early 64-bit Macs were deprecated before 64-bit KEXTs became common. Since those older drivers are 32-bit, Mountain Lion won't load them. We believe Apple decided it was better to draw the line in the sand for some older machines rather than invest the resources into updating the drivers for these older GPUs."